This invention relates to devices for analyzing hydrophilic contact lenses (also known as soft contact lenses) and, more particularly, to a method of measuring the sagittal depth and center thickness of a lens, as well as a device for accomplishing this. The device also permits measurement of lens diameter and inspection for defects.
With the increasing popularity of soft contact lenses, increasing numbers of manufacturers have entered the field. In addition, there is an increasing number of optometrists, opthalmologists and other practitioners involved in fitting these lenses on a patient. The large volume of lenses being manufactured has also created a large quality control problem, not only for the manufacturers but also for the individual practitioners. Each lens must be accurately dimensioned and be free of scratches, nicks and other similar defects to provide a comfortable and physiologically proper fit. This invention is directed primarily to the practitioners who actually fit lenses on their patients, although it is useful to anyone who desires to check the quality and certain key dimensions of a soft contact lens.
Lens quality is evaluated by visual inspection. Due to the nature of refractive materials, the visibility of lens surface irregularities (such as scratches or engraved codes) and lens matrix defects (such as small fractures) varies with the angle of incidence of the illuminating light. Whereas light from one particular direction might be brightly reflected or refracted toward the observer by an irregularity, a different angle of incidence might not have the same effect, so that the irregularity would be difficult or impossible to see. Moving the lens in relation to the light source enables the operator to examine the lens with several different angles of incident light. Prior art devices utilizing static projection systems hold the lens and light source in fixed positions, allowing only one angle of incident light.
One important dimension of a contact lens is its radius of curvature, which is a factor in the prescription of a contact lens. The radius of curvature can be determined by measuring the chord sagittal depth and calculating radius. Because soft lenses become aspheric when hydrated, the calculated radius is an approximation. A better method of determining curvature is to measure the full sagittal depth, or sagittal depth (or height) of the lens, sometimes also referred to as the "vault", of a lens. This is the distance between the center of the plane containing the circular bottom edge of the lens and the vertical projection of that center on the concave lens surface. In addition to central lens curvature, full sagittal depth measurements include the lens fitting influences of other lens dimensions; primarily diameter, but also optic zone size, peripheral curves, edge bevels and additional peripheral asphericty induced by lenticularization and edge configuration. Most previous lens analyzers do not directly measure the full sagittal depth but instead measure the chord sagittal depth. The device described in United Kingdom Pat. No. 1,427,030 to Highgate, for example, measures the distance between the plane of intersection of the lens support device with the concave surface of the lens, and the projection of the center of that plane on the lens surface. Since the lens is not supported on its bottom edge but is instead supported at points above the bottom edge, the resulting measurement is actually shorter than the full sagittal depth, ignoring the important peripheral lens geometry. This lessens the usefulness of the measurement. Other devices, such as those described in U.S. Pat. Nos. 4,171,576 and 4,212,107, use complex, expensive or time consuming light projection or electronic systems to measure chord sagittal depth.
Another important lens dimension essential to a proper fit and to proper physiological effect is the center thickness of the lens. Prior devices, such as the system described in U.S. Pat. No. 3,917,391 and the Model JCF analyser from Optimec Services, Ltd., require elaborate, expensive, and heat producing light projection systems coupled with a fan and heating element to maintain the hydrophilic lens and the bath in which it is kept at the proper temperature while measurements are taken. This is obviously complicated and expensive as well as difficult to operate.
To accomplish these measurements, it is necessary to carefully position the lens on a support so that an accurate reading can be obtained. Placing the delicate lens in previously designed instruments involves manipulating the lens to avoid air bubble retention and careful positioning of the lens on a pedestal. Tweezers are normally required to accomplish this, despite the admonition from several lens manufacturers to avoid their use. Soft contact lenses vary in their durability, and some are extremely susceptible to damage from tweezers. This is particularly true if the lens is dry during the measurement process, as is true with many prior art procedures. When the lens is dry, it loses some flexibility and tears more easily.
A positioning problem associated with the lens while measurements are being taken, particularly when a measuring rod or probe is to be used as in U.K. Pat. No. 1,427,118, is the centering of the lens over the probe. Unless the probe is positioned along the central radius of the lens, a true measurement of the sagittal depth and center thickness will not be obtained.
Another problem associated with devices using a probe to measure sagittal depth is the difficulty in detecting the initial lens movement occuring as the probe first touches the concave lens surface. Prior art discussions in U.S. Pat. Nos. 4,171,576 and 4,212,107 refer to the difficulty in detecting this initial movement, even with microscopes and viewers.
Still another problem associated with the use of pedestal lens supports in wet cells is the tendency of the immersed, lightweight lens to float off of the pedestal.
Accordingly, an object of this invention is to provide a method and apparatus for measuring the diameter, full sagittal depth and center thickness of a hydrophilic contact lens in its hydrated state. Another object is to provide such a method and apparatus so that the resulting measurements are more accurate than many prior art devices and yet can accomplish accurate measurements at a fraction of the cost and time previously required. A further object is to provide an apparatus which also permits detailed visual inspection of a lens using various angles of incident light. Yet another object is to manipulate the lens for immersion, bubble venting, centering, measuring, and retrieval without risk of damaging it.